No abstract

Abstract. Arctic warming and permafrost degradation are modifying northern ecosystems through changes in microtopography, soil water dynamics, nutrient availability, and vegetation succession. Upon permafrost degradation, the release of deep stores of nutrients, such as nitrogen and phosphorus, from newly thawed permafrost stimulates Arctic vegetation production. More specifically, wetter lowlands show an increase in sedges (as part of graminoids), whereas drier uplands favor shrub expansion. These shifts in the composition of vegetation may influence local mineral element cycling through litter production. In this study, we evaluate the influence of permafrost degradation on mineral element foliar stocks and potential annual fluxes upon litterfall. We measured the foliar elemental composition (Al, Ca, Fe, K, Mn, P, S, Si, and Zn) of ∼ 500 samples of typical tundra plant species from two contrasting Alaskan tundra sites, i.e., an experimental sedge-dominated site (Carbon in Permafrost Experimental Heating Research, CiPEHR) and natural shrub-dominated site (Gradient). The foliar concentration of these mineral elements was species specific, with sedge leaves having relatively high Si concentration and shrub leaves having relatively high Ca and Mn concentrations. Therefore, changes in the species biomass composition of the Arctic tundra in response to permafrost thaw are expected to be the main factors that dictate changes in elemental composition of foliar stocks and maximum potential foliar fluxes upon litterfall. We observed an increase in the mineral element foliar stocks and potential annual litterfall fluxes, with Si increasing with sedge expansion in wetter sites (CiPEHR), and Ca and Mn increasing with shrub expansion in drier sites (Gradient). Consequently, we expect that sedge and shrub expansion upon permafrost thaw will lead to changes in litter elemental composition and therefore affect nutrient cycling across the sub-Arctic tundra with potential implications for further vegetation succession.

1. Thawing permafrost in northern latitudes has led to deepening active soil layers and fluctuating water tables. This could increase plant access to permafrostderived nitrogen (N), phosphorus (P) and other nutrients such as calcium (Ca) and magnesium (Mg), and subsequently increase plant productivity and ecosystem carbon storage and nutrient cycling. We hypothesized that deepening permafrost thaw and water table fluctuations would alter species-specific foliar N:P ratios. Since there is often more P, Ca and Mg available in the deeper mineral soil layers and more N available in the shallow organic layers, we expected that deeply rooted species would decrease foliar N:P ratios due to root proximity to thawing mineral soil, and plants with shallower rooting systems mostly in the organic layer would increase foliar N:P ratios.2. We assessed foliar and canopy nutrient responses of seven vascular plant species in moist acidic tussock tundra vegetation in the northern foothills of the Alaska Range to variable soil thaw depths and water table levels induced by either a natural thermokarst gradient or a winter warming snow fence experiment.3. In both the natural thermokarst gradient and the warming experiment, wet or deeply thawed areas generally led to an increase in foliar nutrient concentrations and greater canopy mass and canopy nutrients. For the majority of species, foliar N:P ratios remained proportional or decreased in deciduous species in wet sites, with the exception of one shallowly rooted species that increased foliar N:P ratio in deeply thawed sites. Overall, plant acquisition of P was more related to water table level than to thaw depth, and water table modulated the canopy biomass response of the species at the warming experiment. 4. Synthesis. Foliar N:P ratios suggest that plant species in this tussock tundra ecosystem are either remaining or becoming more N-limited as thaw depth deepens and water table level rises, indicating that P is not likely to become the primary limiting nutrient with the progression of permafrost thaw. However, the amount of deeply thawed, wet areas that develop on the landscape as permafrost thaws will be important contributors in the total movement of nutrients above-ground.

No abstract

Abstract. Arctic warming and permafrost degradation are modifying northern ecosystems through changes in microtopography, soil water dynamics, nutrient availability, and vegetation succession. Upon permafrost degradation, the release of deep stores of nutrients such as nitrogen and phosphorus from newly thawed permafrost stimulates Arctic vegetation production. More specifically, wetter lowlands show an increase in sedges (as part of graminoids), whereas drier uplands favor shrub expansion. In turn, shifts in the composition of vegetation may influence local mineral element cycling through litter production. In this study, we evaluate the influence of permafrost degradation on mineral element foliar stocks and potential annual fluxes upon litterfall. We measured the foliar elemental composition (Al, Ca, Fe, K, Mn, P, S, Si, and Zn) on ~500 samples of typical tundra vegetation species from two contrasting Alaskan sites, i.e., under experimental (CiPEHR) and ambient (Gradient) warming. The foliar concentration of these mineral elements was species specific, with sedge leaves having relatively high Si concentration, and shrub leaves having relatively high Ca and Mn concentrations. Therefore, changes in the species biomass composition of the Arctic tundra in response to permafrost thaw are expected to be the main factors that dictate changes in elemental composition of foliar stocks and maximum potential foliar fluxes upon litterfall. We observed an increase in the mineral element foliar stocks and potential annual litterfall fluxes, with Si increasing with sedge expansion in wetter sites (CiPEHR), and Ca and Mn increasing with shrub expansion in drier sites (Gradient). Consequently, we expect that sedge and shrub expansion upon permafrost thaw will lead to changes in litter elemental composition, and affect nutrient cycling across the sub-Arctic tundra, with potential implications for further vegetation succession.